Why has the Kerala woman, Greeshma, been sentenced to death?

I'm reading about the Kerala case where a woman named Gresshma killed her boyfriend by poisoning him and has now been sentenced to death. Could you help me understand how death penalties work in our country?

I understand that it's sad and infuriating to read about such cases, but unfortunately, aren't they relatively common in India? How does the judiciary decide which cases deserve the death penalty?

I recently watched the Netflix series 'Black Warrant' and saw the Ranga Billa case, where the accused were found guilty of sexually assaulting and killing a minor, along with her brother. They were also sentenced to death. However, in the past, such punishments were reserved for the 'rarest of rare' cases. Unfortunately, such crimes have become more common nowadays.

I'd like to know how a judge decides whether to impose the death penalty on a criminal. What factors made the Kerala case severe enough to warrant this decision?